Manufacture of coated paper and product



United States Patent Ofi 2,700,621 Patented Jan. 25, 1955 iceMANUFACTURE OF COATED PAPER AND PRODUCT Hellmuth C. Schwalbe, Robert C.Hydell, and James J. OConnor, Chillicothe, Ohio, assignors to The MeadCorporation, Dayton, Ohio, a corporation of Ohio No Drawing. ApplicationJanuary 6, 1950, Serial No. 137,278

7 Claims. (Cl. 11765) This invention relates to the manufacture ofcoated paper and more particularly to mineral coated paper adapted tohave surface characteristics suitable for receiving fine printedimpressions such as are used in 'high quality magazine printing of finehalf-tone and multicolor reproductions.

Generally in the manufacture of coated paper of the character describeda coating material comprising a mineral pigment and an adhesive orbinding material in Water suspension is applied to the surface of thepaper by suitable coating apparatus. The pending application of OConnor,Savage and Schwalbe, Serial No. 774,294,

now Patent No. 2,565,260, shows roll coating apparatus with which asuitable finish imparting surface coating may be applied to paper toform a satisfactory smooth surface thereon with characteristics adaptedfor high quality and exacting printing requirements demanded for thefaithful reproduction of fine half-tone and multicolor illustrations inmagazines and the like but utilizing coating material comprisinggenerally a highly thixotropic aqueous suspension with relatively highsolids content of a mineral pigment and adhesive material such as vcasein or modified starch.

As attempts are made to increase the speed of operation of such rollcoating apparatus, it is desirable to apply coating material from asuspension the solids content of which is as great as may be practicabletoachieve a finished coated product with suitably formed surfacecharacteristics. That is to say, with such high speed roll coatingapparatus, the application of desirably heavy coat weights of the orderof ten pounds or more per side per ream of five hundred sheets 25 x 38inches will be facilitated if the mineral-adhesive coating suspensionsapplied to the paper have solids contents of substantially more than ofthe order of As the speed of travel of the paper web through the coatingapparatus is increased, a greater quantity of coating suspension must beintroduced to the paper web per unit of time in order to apply adesirably heavy coat Weight. If it is attempted, however, to increasethe solids content of such mineraladhesive suspensions as heretoforeused, such coating suspenslons rapldly become more and more viscousuntil a they reach a state wherein a further increase in solids contentof but a few per cent will produce such a substantial and drasticincrease in the viscosity of the suspension that regular roll coatingapparatus will not adequately apply and meter and acteristics to thecoated paper in high speed commercial operations with such roll coatingapparatus and techniques. One solution of this problemis shown in theabove mentioned copending application of applicants assignee where itwas the "characteristics of the coating suspension it could be madehighly thixotropic so that, notwithstanding the greatly increasedviscosities resulting from increasing the solids content substantiallyabove what had been considered practicable, working in shear, the highlythixotropic characteristic of the coating suspension renders ittemporarily less viscous for a sufiicient time to permit its ready andsatisfactory application to the paper. suspensions of this character,however, such a drastic rise in viscosity may occur when relatively highsolids contents are increased even a few per cent, e. g., in the rangeof 56% to 60% solids, to an extent that above such heretofore workableEven with coating material uniformly distribute the coating material togive suitably smooth surface charfound that by properly controlling isions become so viscous and substantially nonflowable, even thoughhighly thixotropic, that it is impracticable to get sufficient workingin shear within the limitations of the power and strength of the rollcoating apparatus to reduce the viscosity sufiiciently to besatisfactorily fiowable and readily measured and applied to the paper togive desired results.

It has now been discovered, however, that, by the present invention, asuitable coating material suspension may be readily applied to paper,using existing high speed roll coating apparatus, with solids content ashigh as approximately 65% to 80% or more yet with initial viscosities(as determined with a MacMichael viscosimeter with a cup diameter of2%", a bob diameter of 1 and thickness of A" and clearance from thebottom of at 40 R. P. M.) of as low as 5 to 20 poises or well within therange of practicable coating material viscosity limits of such existinghigh speed commercial roll coating apparatus. This advantageous coatingoperation is accomplished hereunder by using materials such as urea andstarch adhesive materials hereinafter described in combination withsuitable additional components such as highly dispersed mineral pigmentsand nonbulking lubricants which give relatively reduced viscosity sothat the relatively high solids content colors have such normalviscosity and flowabilityas to be readily usable in such roll coatingapparatus.

A principal object of this invention is to provide for the manufactureof coated paper of the character described using finely divided finishimparting mineral coatings which may be applied to the paper in the formof aqueous suspension with high solids content yet low MacMichaelviscosities.

Another object of this invention is to provide a method of coating paperof the character described using coating material suspensions whichpossess substantially higher solids content than has heretofore beenpossible with other satisfactory coating materials of comparably lowMacMichael viscosity.

Still another object of this invention is to provide a method of coatingpaper of the character described using coating material suspensionswhich possess substantially lower initial MacMichael viscosities thanhas heretofore been possible for satisfactory coating materials ofcomparably high solids content.

A further object of this invention is to provide a coating materialsuitable for application to paper by high speed roll coating apparatusand techniques to produce a heavier coat weight applied to such paperthan has heretofore been possible on existing high speed apparatusandwith superior smooth surface characteristics adapted for fine highquality printing.

A further object of this invention is to provide a coating material ofthe character described which will resist excessive adsorption into thepaper sheet or web to which it is applied notwithstanding a low initialMacMichael viscosity so that substantially all of the coating materialmay be retained upon the surface of the paper to perform its smoothsurface providing function thereon.

Another object of this invention is to provide a suitable coatingmaterial of the character described adaptable for convenient andeconomic use and application with existing paper coating apparatusandmethods within the intended range of operationthereof.

A still further object of thisinvention is to provide a coated paperproduct of the character described to which has been imparted adesirably smooth surface notwithstanding the application of a heavy coatweight of high solids coating material compositions.

Still another object of thisinvention is to provide a coating materialof the character described such that, when applied to the paper anddried, the applied surface coating will possess suitable plasticcharacteristics to admit of readily obtaining thereon or impartingthereto extremely smooth surface characteristics upon supercalenderingof the applied coating.

Still another object of this invention is to provide a paper coatingprocess of the character described in which the variable characteristicsof viscosity of the coating suspension, readily practicable solidscontent thereof, and applied coat weight and wet fihn thickness may beso =3) coordinated one to the other as to arrive at desirably optimumcombinations of such variable characteristics for coating materials ofdifierent compositions and conforming to the requirements of papercoating applications, apparatus and problems.

Still a further object of this invention is to provide for use in apaper coating process of the character described an improved coatingmaterial suspension in which the properties of viscosity and solidscontent are so adjusted to give very low or the lowest possible lowshear viscosity concurrently with maximum mineral and adhesive coatingsolids content with a substantially low high shear viscosity.

Other objects and advantages of this invention will be apparent from thefollowing description and the appended c aims.

The high solids contents of coating material suspensions used inaccordance with this invention facilitate the manufacture of coatedpaper with desired surface characteristics by resisting excessivepenetration of the coating suspension into the paper so thatsubstantially all of the mineral coating material remains effectivelyupon the surface of the paper to provide the desired smooth mineralcoating thereon; yet the low viscosities of these coating suspensionsgreatly facilitate the pumping and other handling and introduction ofthe coating material suspension to the coating apparatus as well asfacilitating the smooth application of a fluid coating film to the paperwithout the undesirable formation of irregular surface characteristicsobtained from excessively viscous coating material. Furthermore coatingmaterial suspensions of this invention possess what might be termed aself-plasticizing action, as hereinafter described, whereby coated paperproduced in accordance with this invention admits of much moresatisfactory surface smoothing as by supercalendering even after theapplication and drying of the coated material. In contrast to relativelyhigh solids and highly thixotropic coating suspensions as heretoforeused, the viscosities of the coating material suspensions of thisinvention vary within a comparatively narrow range for a given solidscontent range under normal working conditions well within applying andhandling limits of high speed roll coating apparatus, such as isdescribed in the above mentioned copending application of applicantsassignee, when used in its normal and intended manner.

As illustrative of this astonishing advance embodied in this invention,it should be noted that coating material compositions hereof, ashereinafter specified. having a solids content of as high as 75.5 andMacMichael viscosities of as low as 19 poises have been applied withexisting roll coating apparatus to obtain a coated paper productcarrying a coat weight of as much as seventeen pounds per ream per sidewith a desirable surface smoothness comparable to that heretoforeobtainable on high speed roll coating apparatus with but ten pounds orless of coating per side-an increase of as much as 70% in weight ofcoating of satisfactory high quality characteristics over proceduresheretofore used.

While this invention has so far been found of greatest commercialadvantage when the solids content of the coating material suspension isin the high solids range of 65% to 70% solids or even higher,nevertheless in the lower ranges, such as 50% ing difficulties forcertain desired coated products have heretofore limited satisfactory useof nonthixotropic suspensions, more desirable and satisfactory heavilycoated paper is secured since lower viscosity of the coating suspensionsembodying this invention will also permit of more uniformly applied andmore readily distributed and smoother surface coatings. That is to say,with coating compositions as heretofore used, the viscosity of thecoating suspension so increased (in certain ranges of solids content)that a point was reached at which the coating suspension was so viscousthat its application to the paper by the coating applying rolls of theroll coating apparatus .resulted in the formation of irregular surfacecharacteristics such as so-called piling or worming evidenced by theformation .of sharp spike-like peaks and sharp crater-like valleys inthe coating surface.

In'this invention .it has been discovered that high solids content, lowviscosity coating material suspensions may be suitably applied usingroll coating apparatus vsuch .as is shown and described in the abovementioned pending application of applicants assignee. One such coatingcomposition with which satisfactory results have been obtained maycontain the following formulation:

4 parts Stayco M 8 parts Sweetose 15 parts calcium carbonate 73 parts S.M. C. Clay 1 part Calgon 0.25 part calcium stearate 10 parts urea Theabove coating composition may be admixed with Water to provide anaqueous coating material suspension having total solids content of 7.5%and, when so admixed to give this extremely high solids content, theMacMichael viscosity at R. P. M. of the coating composition suspensionwill be 19 poises. Such a suspension may be applied to a raw stock paperweb comprising essentially 30% pine pulp, 31% sulphite pulp, 27% sodapulp and 12% sulphite broke at web speeds of approximately 600 feet perminute to give a coat weight of approximately 17 pounds per ream on oneside with sati sfa'ctory surface smoothness characteristics using rollcoating apparatus of the character described with a metering pressure of200 pounds per linear inch.

In the formula set out above, the S. M. C. Clay is a finely dividedmineral clay pigment suitably processed for use in the paper industryand manufactured by Georgia Kaolin Co. Both such finely divided claysand the calcium carbonate noted are selected for inclusion in thecoating compositions of this invention because their highly dispersed orgreatly deflocculated nature diminishes'as much as practicable'thecontribution made by the pigment suspension to the overall viscosity ofthe coating composition. Calgon is a commonly accepted trade name for asodium polyphosphate developed for water conditioning. In water solutionit disperses finely divided metal oxides and salts, and inhibitscrystallization of slightly soluble combinations such as calcium tosolids, where operatv carbonate, and is used in this composition as adispersing agent. Calcium stearate is here used as a lubricant. Thisparticular lubricant is selected as being a so-called nonbulkinglubricant instead of the more generally used viscosity increasing soaplubricant in order to further minimize the viscosity contributions ofthe various components of the coating suspension to the viscosity of thewhole suspension. Stayco M is a trade name for a thin boiling oxidizedstarch adhesive material made by the Staley Manufacturing Company, andSweetose" is a syrupy starch product converted or modified ashereinafter described to a greater degree than the Stayco M referred toand is here used as part of the adhesive component of the coatingmaterial.

The terms modified or converted starch indicate very generally a starchcolloidal suspension or sol which has been oxidized, as by reaction withs are such suitable chemical as sodium hypochlorite, or has been acted.upon by such liquefying enzymes as, for example alpha amylase by methodsof starch adhesive material manufacture welllgnown in the coated paperindustry. Starch adhesive materials may be so modified or converted tovarious degrees by controlling the severity or completeness of suchconverting or modifying treatment. The extent or severity of suchconverting treatment tends to modify considerably both the viscosity ofthe starch sol and its adhesive properties, a greater extent ofconversion tending to decrease both the viscosity and the adhesivestrength.

As indicated in the above formulation. the properties of the adhesivestrength and viscosity of the starch com ponent of the coatingcompositions are controlled by admixing a slightly converted andrelatively adhesive starch such as the Stayco M with a highly converted,highly fiuid starch product of lower adhesive strength such as theSweetose so that the combination thereof will give a starch adhesivematerial component of adequately high adhesive strength yet adequatelyloW viscosity. Satisfactory results have been obtained with the aboveformulation using as the less highly converted starch sol one which mayhave a so-called high shear viscosity (as measured as disclosed in thepatent to Gricsheirner No. 2,392,662) of centipoises at 14 F. when thestarch solids content is approximately 31%, and. as the fluid starchcomponent, astarch ,sol with a high shear viscosity of 15 ccntipoises atF. with starch solids content of approximately 31%. Therefore, as in thecoating com position set forth above, satisfactory results have beenobtained by using a combination of fluidstarch" with a somewhat lessconverted or binding material which will contribute as little aspossible to the over-all viscosity of the whole coating materialsuspension and yet will provide adequate adhesive or binding propertiesto cause the mineral pigment in the coating material to adheresatisfactorily to the surface of the paper to be coated.

The astonishing combination of such heretofore believed conflictingproperties as such usable'high solids content ranges with suchconveniently low MacMichael viscosities is attributable in part, it isbelieved, to the inclusion in coating composition of this invention ofthe component urea set forth above in combination with the use of ahighly converted or fluid starch adhesive material. The actual mechanicswhereby the coating compositions set forth herein achieve theirphenomenal results when used with this invention, however, are not nowcompletely understood.

The fluidity of mineral-starch coating suspensions may be increased(i.e. the viscosity decreased) by subjecting the starch adhesivematerial to converting operations as noted above, and that suchconverting of the starch will also decrease the adhesive properties ofthe starch as noted. It is believed that, within limits, the increasedsolids content of these coating suspensions inhibits the penetration ofthe fluid suspension into the fibrous paper sheet so that, with highsolids content suspensions, more adhesive material remains effectivelyin and upon the surface zone of the paper to accomplish the pigmentbinding function. Consequently, notwithstanding the decreased adhesiveproperties of more highly converted starch, actually less starchadhesive material is needed in the original suspension to hold a givenquantity of pigment upon the surface of the paper with compositions ofthis invention.

Furthermore it is believed that the inclusion of urea in such a coatingmaterial suspension causes some physical, chemical or mechanicalcombination or interaction to occur between the urea and the starchadhesive material whereby the viscosity of the suspension is loweredperhaps because the extremely hydrophilic starch sol is made to releaseeffectively more water for dilution or viscosity decreasing action. Thedecreased viscosity effect of converting the starch itself may indeed bepartially due to the action of such converting treatment tending tobreak down many of the entangled branches and side chains of the highlycomplicated starch molecules so that the highly converted starchapparently has less water withholding characteristics, so to speak, sothat a smaller amount of water may be used in the coating suspension toprovide adequate flowability. Such an effect is substantially increasedby the use of urea so that more of the water present in the coatingsuspension is available for dilution or viscosity decreasing action.

Also the inclusion of urea in such starch-mineral-water suspensions hasthe effect of actually, increasing the amount of solid coating materialleft deposited upon the paper surface after the application and dryingof the coating suspension while actually decreasing the normal initialviscosity of the original aqueous suspension yet without appreciablyincreasing the volume of diluent or the liquid phase of the coatingsuspension. This peculiar effect has the double advantage of makingusefully avail able a greater quantity of ultimately solid materialtoproduce the finished coating at greatly decreased viscosities while yetavoiding disadvantages heretofore attended upon the use of highly fluidcoating materials with high speed roll coating apparatus of thecharacter described. That is to say the penetrating into or absorptionby the paper of the highly fluid coating suspension is inhibited by thehigh solids content thereof as well as the tendency heretofore of highlyfluid suspensions to splash or be cast off the coating applying rolls ofthe apparatus by the action of centrifugal force to fall upon thealready coated paper web and form irregularities thereon. I

It is pointed out, however, that whether or not the above hypotheses becorrect is not necessarily a determining factor in practicing thisinvention. It has been discovered, in any event, that a smallerpercentage of water will produce added fluidity without highlythixotropic characteristics for a given high solids contentif a coatingmaterial suspension is used in accordance with this starch to provide anadhesive invention containing urea and that the advantageous combinationof high solids content with low viscosity may be further enhanced byalso using in such coating material suspensions a highly converted orfluid starch adhesive material. Consequently, it is now no longernecessary to depend 'upon highly thixotropic coating suspensions toobtain desirably high solids contents for the high speed commercialapplication of desirably heavy coat weights with suitably formed surfacecharacteristics utilizing existing roll coating apparatus operated inits normal and intended manner. Thus, with these coating compositionscontaining urea, the mineral-starch suspension is no more dilute (in thesense that the ratio of water to suspended pigment is no greater) whenurea is included in the coating composition, yet the presence of ureayields a greater quantity of solids than-has heretofore been obtainablefrom a given amount of coating suspension of comparably low viscosity asheretofore used.

Accordingly, for satisfactory high speed coating results with heavy coatweights, coating material compositions may satisfactorily be produced inaccordance with this invention so that extremely high solids contentsare obtainable before the point at which the viscosities of coatingsuspensions undergo the sudden and drastic increase referred to above.

Additional coating composition formulations embodying this invention maybe produced as set forth below:

Example 1 11 parts Stayco M 12 parts calcium carbonate 77 parts 81 Huberclay 0.5 part pyro phosphate 0.25 part calcium stearate 10.0 parts ureaThe above formula when admixed with water to give a coating compositionsuspension of 65.0% solids had a MacMichael viscosity of 40 R. P. M. of32.2 poises. It was applied toa raw stock paper comprising 30% pinepulp, 31% sulphite pulp, 27% soda pulp, and 12% sulphite broke at aspeed of 625 feet per minute on conventional apparatus to give a coatweight of 14.4 pounds on one side.

- Example 2 7.0 parts Stayco M 4.0 parts Sweetose 10 parts calciumcarbonate 79 parts S. M. C. Clay 1.0 part Calgon 0.25 pa-rt calciumstearate 10 parts urea Satisfactory results have been obtained from theabove composition when admixed with Water to give a coating materialsuspension having a solids content of 70.8% and a MacMichael viscosityat 40 R. P. M. of 24 poises. Such composition, when coated on a paperraw stock comprising approximately the same composition as set forth forExample 1 above at approximately 600 feet per minute produced a coatweight of 11 pounds for one side of eX- tremely'satisfactory smoothnesseven before calendering.

Quite apart from the unusually high solids content obtainable with thecoating compositions of this invention, the lower viscosities availableas Well as the fact that coating compositions of this invention arequite readily calendered with supercalendering after having been driedupon the paper all combine to make the compositions of this inventionuseful for attaining highly satisfactory coating results even whensuch'unusually high solids content are not desired or required. In thisconnection it should be noted that it has also been discovered that thesolids material deposited upon the surface of the paper sheet'or webcoated according to this invention and after the Water has been removedas by drying possesses the characteristic .of apparently being somewhatplasticized so that it is readily adaptable to the smoothing act-ion ofa supercalender. Although the exact mechanics of this phenomenon are notunderstood, it is believed that the inclusion of urea in the coatingcompositions of this inventron tends to impart plasticity to the driedcoating so that smoothing and finishing action of the supercalender Suchcoating composition,

In this'connection it may be well to note that coating materialcompositions formulated in accordance with this invention may bethixotropic to a certain degree. That is to say, although theirviscosities may not undergo the wide variations common to highlythixotropic colors as heretofore used, some such thixotropic variation,although to a substantially small degree, may be present. Although anythixotr-opy which may be present in the formulations of this inventionmay well be utilized as desired, the characteristics and surprisingresults obtainable from coating materials of this invention render it nolonger necessary to depend upon nicely controlled thixotropy in order toobtain satisfactory coating results with high solids contentmineral-starch coating suspensions.

If however, the coating applying apparatus to be used is so constructedas to take advantage of or to be able to utilize thixotropic coatingcompositions, coating suspensions embodying this invention may beformulated for highly satisfactory use on such apparatus. One suchexample of a coating composition embodying this invention and formulatedfor use upon high speed coating apparatus designed to utilize highlythixotropic colors is as follows:

Example 3 10.5 parts starch (converted to 90 centipoises high shearviscosity at 140 F. and 31% solids) 10.0 parts calcium carbonate 74.5parts S. M. C. Clay 5.0 parts urea 0.8 part Calgon 0.7 part soap 0.1part soda ash when admixed with water to form a coating compositionsuspension having 70% solids content, exhibits an initial MacMichaelviscosity of from 75 to 90 poises. Such a suspension has beensatisfactorily applied to a paper raw stock comprising 14.5% sulphitepulp, 35.5% bleached kraft pulp, 50.0% groundwood at a speed of 950 feetper minute to give a coat weight of 14 pounds on one side and 15.5pounds on the other side.

Another such coating composition embodying this invention is formulatedas follows:

Example 4 11.5 parts starch (converted to 174 centopoises high shearviscosity at 140 F. and 31% solids) 26.5 parts calcium carbonate 67parts Stellar Clay (manufactured by Edgar Brothers Company) 0.5 partsoap 0.5 part pyro phosphate 10.0 parts urea Such formulation, whenadmixed with water to give a coating composition suspension having 58.5to 60.9% solids, has a viscosity of approximately 79 poises and producessatisfactory results with coat weights from 19.5 to 21.9 pounds. Suchapplication should be contrasted with coat weights obtained of less than17 pounds using colors not formulated according to this inventionwherein the highest solids contents available was some lower than thatgiven above to produce comparable finished coated products on the samehigh speed apparatus.

It will be noted in the coating compositions above set forth that thequantity of mineral pigment utilized in each case remains approximatelyof the same order of magnitude. The actual quantity of adhesivenecessary to cause adhesion and cohesion and binding to the surface ofthe paper of such quantities of mineral pigmented material depends uponthe degree of conversion which such adhesive has undergone as well asthe characteristic desired to be imparted to coated paper.

The specific ranges of amounts of components in the i above formulationssuch as dispersing agents and lubricants, etc., are quite readilydetermined according to principles well-known in the art of coatingpaper. It should be noted, however, as pointed out above, that theviscosity of the final complete coating composition suspension will notbe increased in a non'bulking lubricant such as calcium stearate orcastor oil and hexylene glycol be used insteadof soap. Coatingcompositions embodying this invention may contain urea in an amount fromapproximately 3% to 20%: of the total solids of the coating composition, although urea in amounts of from 5% to 12% are preferred andhave been found satisfactory to give highly satisfactory results in highspeed commercml manufacture. Satisfactory pigment-adhesive ratiosaccording to this invention include from to 95 parts by weight ofpigment to 8 to 15 parts by weight of starch, with ratios in the rangeof to parts by weight pigment to 10 to 13 parts by weight of starchbeing preferred. Satisfactory results have been obtained withcompositions according to this invention having a low shear viscosity ofless than poises MacMichael with about 15 to 40 poises being preferred.These compositions also include the starch converted as noted above tohave a high shear viscosity corresponding to 35 to 175 centipoisesmeasured at F. and at starch solids content of approximately 31%.

Furthermore highly satisfactory results have been obtained using coatingcompositions and methods embodying this invention in coating applyingoperations in which extremely high solids or extremely heavy coatweights were not required. The coating compositions of this inventionwere found highly satisfactory not only because of the lower viscositiesobtainable even at such lower solids content but also because of thereadily smoothable characteristic of these coating compositions uponbeing supercalendered as has been set forth above. One such coatingcomposition of somewhat lower solids content is as follows:

Example 5 12.75 parts starch (converted to centipoises high shearviscosity at 140 F. and 31% solids) 30.25 parts Huber 81a Clay 26.5parts calcium carbonate 30.5 parts Stellar Clay 0.5 part soap 0.5 partCalgon 0.5 part soda ash 8.3 parts urea Such a color composition, whenadmixed with water to give a. color composition suspension having 56.6%solids, at a viscosity of 31.0 poises, was applied in regular high speedcommercial use to give a coat weight of an average of 10.5 pounds perside. The effect of using a lower percentage of urea is indicatedadditionally by the Example 3 above wherein the utilization of 5% ureaenabled the production of a coating color composition containing 70%solids yet did not lower the MacMichael viscosity below 75 to 90 poises.

It should be noted that viscosity values given hereinbefore have beenobtained using a so-called MacMichael viscosimeter at 40 R. P. M. Suchviscosity readings may be generally categorized as low shearviscosities. It is convenient in carrying out coating proceduresembodying this invention to consider also so-called high shear viscosityof the coating suspension. Such viscosity determinations may be measuredwhile the coating suspension under consideration is being Worked in highshear or undergoing high shearing forces and such high shear viscositiesmay be satisfactorily determined by the use of the viscosimeterdisclosed in the Patent No. 2,392,662 to Griesheimer.

It has been found that, as the solids content of starchmineral coatingsuspensions is increased, the variations in low shear viscosity are notnecessarily parallel to the variations in high shear viscosity. This isparticularly true if the coating composition suspension exhibitspronounced characteristics of thixotropy. With coating proceduresembodying this invention, however, satisfactory comparativedeterminations of coating composition suspension results may beconveniently considered in terms of the high shear viscositycharacteristics.

It has been found, for example, that the high shear viscosity ofstarch-mineral coating composition suspensions increase proportionatelyto increases in solids content over a certain range of solids contentsbut that a point may be reached where high shear viscosity undergoes avery drastic and sudden increase with a very small increase incoatingcomposition solids. Thus the high shear viscosity of a starch-mineralcoating composition suspension in a certainrange of solids content maysuddenly increase quite drastically by. as much as 25% to 30% when thesolids contents are. increased by less than 1%. For satisfactoryfinished results with coating apparatus of the character described, theeffective practicable limit on solids content is marked in various typesof starch-mineral coating composition suspensions by the point at whichthis drastic and sud-den rise occurs in high shear viscosity. As hasbeen pointed out before, this effective limit with coating compositionsembodying this invention may be as high as 75 or 80% solids. a

In attempting to formulate coating compositlon suspensions embodyingthis invention, therefore, this relationship of solids content and lowand'high shear viscosities may conveniently be considered. That is tosay, the compositions may be formulated to obtain the highest solidscontent below that range of solids content at which occurs the drasticand sudden increase in high shear viscosity referred to above, and alsoso that the low shear or MacMichael viscosity of the coating compositionsuspension is sufliciently low to be readily handled, controlled, andapplied by the coating apparatus. It is believed that the low shear orMacMichael viscosity characteristic indicates to a large measu're thesmoothness characteristics obtainable on the applied coatingcomposition; that the solids content, as has been noted, contributes tothe obtaining of desirably heavy coat weights at desirably high speedsas well as avoiding the problems of excessive penetration of the coatinginto the paper sheet; and that the high shear viscosity controls theease of measuring, and applying the coating composition suspension tothe paper.

For satisfactory high speed coating results with desirably smooth heavycoat weights using apparatus of the character described, coatingmaterial compositions may be satisfactorily produced and appliedembodying this invention and having viscosities which .fall below thepoint of sudden drastic increase in viscosity referred to above yethaving such extremely high solids content by considering the correlationof the above factors. Thus, after the amount of mineral pigment materialdesired for application to the surface of the paper has been determinedaccording to principles well-known and understood in the art of highquality coated paper manufacture, the amount of starch adhesive materialin combination with a viscosity reducing agent such as urea andconverted to a degree necessary to produce a coating material suspensionof satisfactory viscosity characteristics may be conveniently determinedwithin the following ranges.

That is to say, satisfactory coating material composition forapplication according to this invention may be such that the variouscomponents such as converted starch, urea, nonbulking lubricants, andhighly dispersed pigments will all cooperate so that the ratio of thehigh shear viscosity of the coating composition suspension to the solidscontent thereof falls within the ranges of from 4 to 1.5 and the ratioof the low shear or MacMichael viscosity of the coating compositionsuspension to the high shear viscosity thereof falls within the range offrom 75 to 5. Thus it is possible to select and correlate for economicalreasons or otherwise such variable factors as the extent of starchconversion, the amount of starch adhesive material needed to causesatisfactory binding of the desired quantity of pigment to the papersurface, the amount of urea and the amount of water necessary to give acoating suspension of the desired characteristics for use on apparatusof the character described to produce a coated paper product with adesirably heavy coat weight of' satisfactory surface smoothnesscharacteristics.

While the processes and products herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise processes and products, and that changesmay be made therein without departing from the scope of the inventionwhich is defined in the appended claims.

What is claimed is:

1. In a method for coating a paper web to provide a surface adapted toreceive printing impressions and utilizing roll coating apparatus of thecharacter described, the steps of preparing an aqueous starch adhesivematerial converted to have a high shear viscosity corresponding to 35 to175 centipoises measured at 140 F. and at a starch solids content of31%, admixing with said adhesive material a deflocculated mineralcoating pigment and urea as a viscosity reducing agent for said adhesivein the ratio of 80 to 95 parts by weight pigment to 3 to parts by weighturea and 8 to 15 parts by weight starch, including in said admixture alubricant and dispersant for said pigment and water to form an aqueouscoating suspension having a total solids content of about 60% to 80% anda low shear viscosity of less than 100 poises MacMichael, supplyingMacMichael, supplying saidsuspension to the rolls of said apparatus,metering and conditioning said suspension with high shear working insaid rolls, applying said suspension to the surface of a moving web. ofpaper as an evenly distributed'a'nd blended coating thereon, maintaining during said-metering and-applying steps the low' and high shearviscosities and solidscontent of said suspen sion such that the ratio ofthe high shear viscosity to the solids content falls within the range of4 to 1.5 and the ratio of'low shear viscosity to high shear viscosityfalls within the range of 75 to 5, and drying said coating on said Webfor the product-ion of smooth surface characteristics thereon.

2. For use in a method of coating printing paper including asupercalendering step and utilizing roll coating apparatus of thecharacter position comprising 80 to parts by weight of mineral coatingpigment, 8 to 15 parts by weight of starch converted to an aqueousadhesive sol having a highshear viscosity of 35 to 175 centipoisesmeasured at140" F. and at 31% starchsolids content, 3 to 20 parts byweight of urea as a viscosity reducing and plasticizing agent, minorportions of a dispersant and a lubricating agent for said pigment, andwater providing in said composition prior to application to paper atotal solids content of about 60% to 80% and a low shear MacMichaelviscosity of less than about poises, said composition being furthercharacterized in that the ratio of the high shear viscosity to the totalsolids content of said composition prior to application to paper fallswithin the range of 4 to 1.5 while the ratio of low shear viscosity tohigh shear viscosity falls within the range of 75 to 5 and said ureais'present in amount efliecting a and smoothing of said coatingcomposition in said supercalendering step after drying on said paper. 1

3. In a method of coating'paper to provide smooth surfacecharacteristics adapted to receiving printing impressions and utilizingroll coating apparatus of the character described, the steps ofsupplying to the rolls of said apparatus an aqueous coating suspensioncomprising mineral coating pigment and a fluid starch adhesive materialin the ratio of 80 to 95 parts by weight of pigment to 8 to 15 parts byweight of starch admixed with water to give a total solids content ofabout 60% to 80%, said suspension including 3 to 20 parts by weight ofurea providing a low shear viscosity of less than 100 poises MacMichael,conditioning and metering and working said suspension in said rolls,applying said suspension to the surface of a traveling web of paper as auniformly distributed and blended coating thereon, maintaining therheological characteristics of said suspension during said metering andapplying steps such that the ratio of high shear viscosity to totalsolids content of said suspension falls within the range of 4 to 1.5 andthe ratio of low shear viscosity to high shear viscosity within therange of 75 to 5, drying said applied coating on said web, andsupercalendering said dried coating on said web, said urea eifectinghygroscopic plasticizing of said coating during said supercalenderingstep producing said smooth surface characteristics.

4. A coated paper product possessing extremely smooth surfacecharacteristics adapted for receiving fine printing lmpressions,comprising paper carrying on the surface thereof a mineral pigmentedcoating in excess of 7 pounds of coating per side per ream of 500 sheets25" by 38", said coating including mineral coating pigment and convertedstarch adhesive and urea in the ratio of 80 to 95 parts by Weightpigment to 3 to 20 parts by weight urea and 8 to 15 parts by weightstarch.

5. In a method for coating a paper web to provide a surface adapted toreceive printing impressions and utilizing roll coating apparatus of thecharacter described, the steps of preparing an aqueous starch adhesivematerial converted to have a high shear viscosity corresponding to 35 to175 centipoises measured at F. and at a starch solids content of 31%,admixing with said adhesive material a deflocculated mineral coatingpigment and urea as a viscosity reducing agent for said adhesive in theratio of 85 to 95 parts by weight pigment to 5 to 15 parts by weighturea and 10 to 13 parts by weight starch, including in said admixture alubricant and dispersant for said pigment and water to form an aqueouscoating sus pension having a total solids content of about 65% to 75%and a low shear viscosity of about 15 to 40 poises said suspension tothe rolls of described, a paper coating com substantial plasticizingsaid apparatus, metering and conditioning said suspension with high.shear working in said rolls, applying said suspension to the surface ofa moving web of paper as an evenly distributed and blended coatingthereon, maintaining during said metering and applying steps the low andhigh shear viscosities and solids content of said suspension such thatthe ratio of the high shear viscosity to the solids content falls withinthe range of 4 to 1.5 and the ratio of loW shear viscosity to high shearviscosity falls within the range of 75 to 5, and drying said coating onsaid web for the production of smooth surface characteristics thereon.

6. In a method of coating paper to provide smooth surfacecharacteristics adapted to receive printing impressions :and utilizingroll coating apparatus of the character described, the steps ofsupplying to the rolls of said appa ratus an aqueous coating suspensioncomprising mineral coating pigmentand a fluid starch adhesive materialin the ratio of 85 to 95 parts by weight pigment to to 13 parts byweight starch admixed with water to give .a total solids content ofabout 65% to 75 said suspension including 5 to 15 parts by weight ureaproviding a low shear viscos'ity of about 15 to 40 porses lviacMichael,conditioning and metering and working said suspension 1n said rolls,applying said suspension to the surface of a traveling web of paper as auniformly distributed and blended coating thereon, maintaining therheological characteristics of said suspension during said metering andapplying steps such that the ratio of high shear viscosity to totalsolids content of said suspension falls within the range of 4 to 1.5 andthe ratio of low shear viscosity to high shear viscosity within therange of to 5, drying said applied coating on said web, andsupercalendering said dried .coating on said web, said urea effectinghygroscopic plasticizing of said coating during said supercalenderingstep producing said smooth surface characteristics.

7. For use in a method of coating printing paper including asupercalend'ering step and utilizing roll coating apparatus of thecharacter described, a paper coating composition comprising to parts byweight of mineral coating pigment, 10 to 13 parts by Weight of starchconverted to an aqueous adhesive sol having a high shear viscosity of 35to 175 centipoises measured at F. and at 31% starch solids content, 5 to15 parts by Weight of urea as a viscosity reducing and plasticizingagent, minor portions of a dispersant and a lubricating agent for saidpigment, and water providing in said composition prior to application topaper a total solids content of about 65% to 75 and a low shearMacMichael viscosity of about 15 to 40 poises, said composition beingfurther characterized in that the ratio of the high shear viscosity tothe total solids content of said composition prior to application topaper falls within the range of 4 to 1.5 while the ratio of low shearviscosity to high shear viscosity falls within the range of 75 to 5 andsaid urea is present in amount effecting a substantial plasticizing andsmoothing of said coating composition in said supercalendering stepafter dry ing on said paper.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Gregory: Uses and Applications of Chemicals and RelatedMaterials, Rheinhold, 1939, page 629.

3. IN A METHOD OF COATING PAPER TO PROVIDE SMOOTH SURFACECHARACTERISTICS ADAPTED TO RECEIVING PRINTING IMPRESSIONS AND UTILIZINGROLL COATING APPARATUS OF THE CHARACTER DESCRIBED, THE STEPS OFSUPPLYING TO THE ROLLS OF SAID APPARATUS AN EQUEOUS COATING SUSPENSIONCOMPRISING MINERAL COATING PIGMENT AND FLUID STARCH ADHESIVE MATERIAL INTHE RATIO OF 80 TO 95 PARTS BY WEIGHT OF PIGMENT 8 TO 15 PARTS BY WEIGHTOF STARCH ADMIXED WITH WATER TO GIVE A TOTAL SOLIDS CONTENT OF ABOUT 60%TO 80%, SAID SUSPENSION INCLUDING 3 TO 20 PARTS BY WEIGHT OF UREAPROVIDING A LOW SHEAR VISCOSITY OF LESS THAN 100 POISES MACMICHAEL,CONDITIONING AND METERING AND WORKING SAID SUSPENSION IN SAID ROLLS,APPLYING SAID SUSPENSION TO THE SURFACE OF A TRAVELING WEB OF PAPER AS AUNIFORMLY DIS TRIBUTED AND BLENDED COATING THEREON, MAINTAINING THERHEOLOGICAL CHARACTERISTICS OF SAID SUSPENSION DURING SAID METERING ANDAPPLYING STEPS SUCH THAT THE RATIO OF HIGH SHEAR VISCOSITY TO TOTALSOLIDS CONTENT OF SAID SUSPENSION FALLS WITHIN THE RANGE OF 4 TO 1.5 ANDTHE RATIO OF LOW SHEAR VISCOSITY TO HIGH SHEAR VISCOSITY WITHIN THERANGE OF 75 TO 5, DRYING SAID APPLIED COATING ON SAID WEB, ANDSUPERCALENDERING SAID DRIED COATING ON SAID WEB, SAID UREA EFFECTINGHYGROSCOPIC PLASTICIZING OF SAID COATING DURING SAID SUPERCALENDERINGSTEP PRODUCING SAID SMOOTH SURFACE CHARACTERISTICS.